1. Field of the Invention
The present invention relates to an optical position detecting unit and an optical coordinate input unit, and particularly, relates to an optical position detecting unit using light emitted from a point light source for detecting the position of the point light source or detecting the direction of incidence of light from the point light source, and an optical coordinate input unit formed by using the optical position detecting unit and, for example, used in a digitizer which is an input means for a computer.
2. Description of the Related Art
Heretofore, PSD (Position Sensitive Light Detector) is known as a unit for detecting position, direction, etc. The PSD is a light sensor by which the position of a spotlight radiated onto a light-receiving surface can be detected as a position on the light-receiving surface. By using the PSD in combination with a point light source, there can be produced a position detecting unit for detecting the position of the existence of the point light source, a direction detecting unit for detecting the direction of incidence of light from the point light source, a distance measuring unit for measuring the distance between the point light source and a body of the detector, and so on, as disclosed in "Production of Distance Detector using PSD", August 1990 issue of Transistor Technique, for example.
JP-A-5-19954 is a prior art literature in which a unit for obtaining position information optically is disclosed. In the unit, while a movable body having a light-emitting element is moved on an operation table having an XY coordinate plane set thereon, light from the light-emitting element of the movable body is converged by optical lenses onto an X light-receiving portion and a Y light-receiving portion provided in the center of an X-direction side of the operation table and in the center of a Y-direction side of the operation table respectively to thereby form images on the respective light-receiving portions so that the X and Y coordinate values of the position designated by the movable body are obtained by using information of image-formation positions in the respective light-receiving portions. Accordingly, the unit has a structure for obtaining position information by using the giving and receiving of the light.
In order to detect the position or the like of the point light source by using the PSD, light from the point light source must be transformed into a spotlight before the light is radiated onto the light-receiving surface for detection. In order to form the spotlight, it is necessary to provide an optical lens by which light is converged to form the image of the point light source. Because accuracy in position detection in the PSD is degraded as the size or diameter of the spotlight radiated onto the light-receiving surface increases, there is required a high-precision optical lens by which a spotlight with a desired size can be formed. From another point of view, in order to perform position detection with high accuracy, the distance between the body of the detector having the light-receiving surface and the point light source is limited to be in a focusing range which allows the spotlight with the desired size to be formed. As described above, the PSD used in the conventional position detecting unit requires an optical lens excellent in converging characteristic, so that the distance from the light-receiving surface for detection to the point light source is limited. Accordingly, not only the PSD is not economical but the PSD is limited extremely in conditions for use thereof.
In position detection with use of the PSD, the position of incidence of the spotlight onto the light-receiving surface is obtained on the basis of predetermined arithmetic operation equations and two current values divisionally picked up from two current terminals which exist respectively at opposite ends of the light-receiving surface for example. The relation between the amounts of the two current values divisionally picked up contains information with respect to the position of incidence of the spotlight. Because the two current values obtained by division are picked up as values proportional to resistance values of semiconductor regions which form current paths, it is necessary that the resistance values are distributed uniformly in the semiconductor regions of the current paths in order to obtain two split current values accurately corresponding to the position of incidence. It is however conventionally difficult to obtain such a uniform distribution of resistance values because of limitation in producing technique. It is therefore general that errors within plus or minus 1 percent or so are unavoidable in linearity of position detection in the conventional PSD. The errors are allowable, for example, in the case where the PSD is used in auto-focusing of a camera. It is however necessary to correct detection signals of the PSD in the case where higher accuracy in position detection is required. Because all PSDs produced must be subjected to this correction, it is very troublesome to correct all PSDs.
As mentioned above, position detection with use of the PSD is based on the two split current values picked up from two current terminals and the predetermined arithmetic operation equations. Accordingly, values of detection signals picked up from the current terminals are analog values, and when, for example, digital processing for the predetermined arithmetic operation containing division is carried out by a computer with use of these analog values, there is a disadvantage in that accordance with the accuracy.
Also in the optical coordinate information output unit disclosed in the aforementioned literature (JP-A-5-19954), it is necessary that light from the light-emitting element is converged onto the light-receiving portions of light-receiving means by using optical lenses to form spotlight images on the light-receiving portions to thereby obtain information with respect to the position designated by the movable body. Accordingly, like the case of the PSD, in this case, there is a disadvantage in that optical lenses excellent in converging characteristic from the point of view of detectable position accuracy are required.
It is therefore a main object of the present invention to provide an optical position detecting unit of very wide application in which image formation with use of optical lenses is not required, so that not only the distance between the detecting unit and a light source can be selected to be in a wide range but the position of the light source, the direction of incidence of light, or the like, can be detected with high accuracy regardless of the wide range and, furthermore, errors in linearity can be reduced.
It is another object of the present invention to provide an optical coordinate input unit using the aforementioned optical position detecting unit, which is high in detection accuracy, cordless, excellent in economy and improved both in handling property and in applicability.